The optimization method for the double-grating flat-field spectrographs which employ two aberration-corrected holographic concave gratings is described. The spectral range of the spectrograph is divided into two parts which are covered by two gratings respectively. The two gratings have the same using geometry and are fabricated with different recording geometries for the correction of aberrations, such as defocus, coma, astigmatism and spherical aberrations. A double-grating flat-field spectrograph with a wavelength range of 200~800 nm and detector length of 25 mm is designed. The spectral resolution performance is compared with a single-grating flat-field spectrograph which is designed following the same specifications. The results indicate that the spectral resolution performance can be greatly improved when the double-grating spectrograph is used.